The present invention relates to a process for manufacturing mixed anhydrides represented by the general formula (I): 
in which:
R1 represents H or CH3; and
R2 represents an alkyl, alkenyl, aryl, alkaryl or aralkyl residue, according to which process an alkali metal (meth)acrylate of general formula (II): 
in which:
R1 is as defined above; and
M is an alkali metal, is reacted with a chloroformate of general formula (III): 
in which R2 is as defined above.
Mixed anhydrides are mild acylating agents, the use of which has been described in peptide synthesis in place of conventional anhydrides 
and acid chlorides 
In audition, the only by-products generated by the mixed anhydrides of formula (I) in the acylation reactions are CO2 and the alcohol R2OH, which are easier to remove than (meth)acrylic acid or HCl.
The processes for synthesizing the non-acrylic mixed anhydrides which have been described in the literature are mostly based on the method described by Vaughan in J. Am. Chem. Soc. 73, 3547, 1951: the mixed anhydride is synthesized by equimolar reaction between an alkyl chloroformate and a tertiary amine carboxylate, at low temperature (generally less than 0xc2x0), in a solvent medium (tetrahydrofuran, acetone, toluene, chloroform, etc.). Besides the need to work in a solvent medium, at very low temperature, one of the main constraints of the process lies in the fact that it is necessary to separate off by filtration the trialkylammonium chloride which precipitates in the medium.
In Bull. Chem. Soc. Jap. 1968, 41, 2521-3, Harada and Kondo describe, among the examples, the synthesis of carboethoxymethacrylic anhydride: 
by reaction between potassium methacrylate and ethyl chloroformate, in a solvent medium (chloroform), at 0xc2x0 C., in the presence of pyridine.
The synthesis of carboethoxymethacrylic anhydride is also described in French patent application FR-A-2 212 340, starting with triethyl-ammonium methacrylate and ethyl chloroformate in a solvent medium (acetonitrile).
The synthesis of non-acrylic mixed anhydrides is described in German patent DE-C-1 133 727 by reaction between a carboxylate and a chloroformate in a chloroformate/carboxylate molar ratio of between 0.95 and 1.10.
The processes described in the literature mostly relate to non-acrylic mixed anhydrides and present, depending on the case, many drawbacks, the chief ones being the use of solvents and the use of amines in stoichiometric amount relative to the carboxylic acid or in catalytic amount. The need to separate out the alkylammonium chloride by filtration also constitutes a drawback.
In most of the processes described in the literature, the reagents (II) and (III) are used in equimolar proportions or between 0.95 and 1.1. Under these conditions, it is possible to obtain a mixed (meth)acrylic anhydride 
containing little symmetrical anhydride 
R1 being as defined above. This is particularly inconvenient when it is desired to synthesize carboxyalkoxy (meth)acrylic anhydride containing very little methacrylic anhydride.
It has been discovered, surprisingly, that these problems can be solved by modifying the conditions of the reaction between compounds (II) and (III), and in particular by carrying out this synthesis with a compound (III)/compound (II) molar ratio of greater than or equal to 1.15.
A subject of the present invention is thus a process for preparing a high-purity mixed (meth)acrylic anhydride (I), by reaction between the abovementioned compounds (II) and (III), characterized in that the said reaction is carried out in an aqueous medium and in the absence of amines, the chloroformate (III)/alkali metal (meth)acrylate (II) molar ratio being at least equal to 1.15.
In general, the reaction is carried out with compounds (II) and (III) in which:
R2 is chosen from C1-C40 alkyl, C2-C40 alkenyl, phenyl, phenyl (C1-C40 alkyl) and (C1-C40 alkyl) phenyl residues; and
M represents Na or K.
The mixed anhydrides of formula (I) in which R2 represents C1-C40 alkyl, such as ethyl, n-propyl, isopropyl, n-butyl or isobutyl, are of particular interest and constitute a family of preferred mixed anhydrides of the invention. These are mild acylating agents which can very advantageously replace methacrylic anhydride or (meth)acryloyl chloride which generate methacrylic acid or hydrochloric acid in acylation reactions.
In the process of the present invention, the chloroformate (III)/alkali metal (meth)acrylate (II) molar ratio can be between 1.15 and 2, preferably between 1.5 and 1.7, in order to limit the formation of by-products of the type 
for example. At and above 1.15, a substantial improvement in the selectivity and yields are observed; thus, when the synthesis of carboethoxymethacrylic anhydride is carried out by reaction between sodium methacrylate and ethyl chloroformate, a reduction in the formation of methacrylic anhydride and by-products is observed. Moreover, a high molar ratio is favourable in terms of selectivity, but is penalizing with respect to the production efficiency.
The reaction according to the present invention is advantageously carried out at a temperature of between xe2x88x9210 and +30xc2x0 C., preferably between +10 and +20xc2x0 C.
In accordance with one specific embodiment of the present invention, the alkali metal (meth)acrylate (II) is prepared in aqueous solution by neutralizing (meth)acrylic acid with the hydroxide MOH, the MOH/(meth)acrylic acid molar ratio being between 1 and 1.5, in particular between 1 and 1.1, and the water/alkali metal (meth)acrylate (II) weight ratio being between 1.5 and 7, in particular between 1.5 and 2, after which the chloroformate (III) is reacted with the alkali metal (meth)acrylate (II).
In accordance with one particularly advantageous characteristic of the present invention, the reaction between the alkali metal (meth)acrylate (II) and the chloroformate (III) is carried out in the presence of a phase-transfer catalyst, dissolved or fixed to a polymeric support such as a styrene-divinylbenzene copolymer or a crosslinked polyvinyl-pyridine resin and used in particular in a proportion of from 0.001 to 0.02 mol, in particular from 0.005 to 0.01 mol, per mole of alkali metal (meth)acrylate (II).
The phase-transfer catalyst is advantageously chosen from quaternary ammonium salts, phosphonium salts and crown ethers.
As examples of quaternary ammonium salts 
mention may be made of those in which:
R4 to R7 each represent C1-C40 alkyl, such as CH3, C2H5, C4H9, C8H17 and C16H33, or aryl such as phenyl, or aralkyl such as benzyl;
X represents one from among Cl, Br, I, OH and HSO4;
and, in particular, tetramethylammonium chloride, benzyltrimethylammonium chloride, benzyltri-n-butylammonium chloride, tetra-n-butylammonium chloride, tetra-n-butylammonium bromide, methyltrioctylammonium bromide and tetra-n-butyl-ammonium hydrogen sulphate;
as examples of phosphonium salts, mention may be made of (C4H9)4P⊕Clxe2x8ax96, (C4H9)4P⊕Brxe2x8ax96, (C8H17)3C2H5P⊕Clxe2x8ax96, (C4H9)3C6H5P⊕Clxe2x8ax96; and
as examples of crown ethers, mention on may be made of 18-crown-6 and dibenzoyl-18-crown-6.
Working in the presence of a phase-transfer catalyst increases the reaction substantially (3 to 5 hours as opposed to 15 hours without catalyst).
The reaction according to the invention is generally carried out with stirring in a thermostatically-controlled jacketed reactor while rigorously controlling the temperature. The crude reaction mixture separates out into two phases by settling if the stirring is stopped (or into three phases if a phase-transfer catalyst is used, fixed to a polymeric support). The reaction progress is monitored by regularly taking samples of the aqueous phase and assaying the residual alkali metal (meth)acrylate (II). The reaction is considered as complete when the degree of conversion of the alkali metal (meth)acrylate (II) is greater than 95%.
At the end of the reaction, the phases of the two-phase or three-phase reaction mixture are separated by settling at room temperature, advantageously washed with water (amount: 20 to 100% of the weight of the organic phase, preferentially 30 to 40%) at room temperature, the organic phase containing the mixed anhydride (I) and the excess chloroformate (III) then being stripped off under vacuum at a temperature of less than or equal to 35xc2x0 C. The mixed anhydride is thus obtained in very high purity.
It is necessary to introduce at least one polymerization inhibitor into the reaction medium, in a proportion of from 500 to 5000 ppm, in particular from 500 to 1000 ppm, relative to the alkali metal (meth)acrylate (II)xe2x80x94or to its precursor (meth)acrylic acidxe2x80x94in order to overstabilize the latter.
As examples of polymerization inhibitors, mention may be made of hydroquinone methyl ether, hydroquinone, phenothiazine and di-tert-butyl-para-cresol.